Technically they aren't immune, they're carriers. They have the virus, but do not suffer from the syptoms. In the Sacrifice comic it was explained that the l4d survivors had been infecting everyone who saved them. If you look carefully at all the people who saved them were somehow exposed to the survivors, with the exception of the couple and Virgel, however they might have been exposed inbetween campaigns. The Finale on The Parish, the helicopter that saved them had the pilot blocked off from the survivors. So my theory is that the l4d2 survivors are being taken to a facilaty and be studied, but will somehow escape and hide away somehow, After awhile the new survivors will come by and meet them.

Well actually, there are the immunes, who are people completely immune to the virus and there are the carriers, who are people who have the virus, but don't suffer from it's symptons, instead infecting anyone who enter in contact whit them. This said, te L4D1 survivors were carriers, but we don't know if the L4D2 survivors were carriers as well or completely immune. But what I think happened is the same thing that happened on L4D1 when the survivors were "saved" by the military. They are locked and used to study the virus, but they somehow escape. But they could also end up dead, who knows? I just think it would be a shame to see 4 survivors who fought across hell, and against the most dangerous things in the world, and survived it all, to die whit a shot intheir head, while being forced against a wall. Bill died, but at least it was a hero death. Well I just hope the same thing that happened whitthe survivors of L4D1 happened to the ones of L4D2. We'll knoe for sure, when they launch L4D3 IF they launch a 3rd game.

If the L4D2 survivors have to go through a Sacrifice like the others, then i wouldn't be surprised if Nick or Coach died to protect the others, most likely Nick, just cause it would seem more appropiate.

Maybe in reference to l4d1 they went to a military facility and got locked up, and then escaped, I can't really see any of them dying or sacrificng themself, it wouldn't fit, if anything they'd blow up a building and walk away from it :pwho knows, maybe we'll see them in l4d2 like we see the l4d1 survivors in the passing?